Analgesic & Anaesthesia For Obstetric Patients Flashcards
Maternal physiology
- O2 consumption + Maternal metabolism
- Maternal tissue: 4 ml/min
- Fetus + Uterus + Placenta: 12 ml/min
- O2 consumption >20 ml/min at term
—> Synthesis of new tissues + Removal of metabolic wastes - Haematological changes
- Physiological anaemia (Red cell volume↑ < Plasma volume↑) —> If at term Hb<11.6, Hct 35.5 —> Then need to think about other causes
- Hypercoagulable state
—> Thrombocytopenia
——> Gestational
——> Immune-mediated
——> **Pre-eclampsia / **HELLP syndrome (Haemolysis, Elevated Liver enzymes, Low Platelets) (much higher risk of bleeding complications / intracerebral haemorrhage)
——> Individual assessment on CI to regional anaesthesia (depend on trend on decrease)
—> Fibrinogen level (normally very high in term women) —> Relatively low level predict severity of PPH —> need Cryoprecipitate / Fibrinogen concentrate transfusion - CVS changes
- CO ↑50%, SV ↑25%, HR ↑25%, SVR ↓20%
CVS examination:
1. Leftward shift of apex beat
2. Accentuation of S1
3. S3, S4
4. Exaggerated physiological split
5. ESM
ECG:
1. Sinus tachycardia (may need to consider PE as a cause as well)
2. LAD up to 15o
3. Ectopic beats
4. T inversion in lead 3, aVF
Significance:
- Hypotension masked until massive blood loss at 30-35% blood volume
—> Need to think about pain / stress / massive bleeding as causes
- Respiratory changes
- FRC ↓20%
—> Reduced O2 reserve but ↑demand (↑minute ventilation + ↑alveolar ventilation)
—> ↑Atelectasis, ↑Shunting, ↑O2 A-a gradient
—> ↓Metabolism + O2 consumption 25% at term
- ↑Ventilation further during delivery (∵ pain, bear down effort, anxiety)
Significance:
- Prone to **Hypoxaemia —> **Quick desaturation —> ***Limited time + attempts of intubation allowed
- GI changes
- ↓Gastric emptying (hormonal, cephalad stomach)
- ↑Gastric acid secretion + acidity
Significance:
- ↑ **Aspiration risk —> **Intubation is required for GA (NO laryngeal mask)
Uteroplacental blood flow
- Major determinant of O2 + nutrient delivery to fetus
- Placenta is a low resistance circuit —> ***Limited autoregulation
- Must avoid aortocaval obstruction at all times —> Left lateral tilting / Manual displacement of uterus
Maternal pharmacology
- Infant exposure dose
- D = C x M/Pauc x V
- D: Infant exposure dose
- C: Maternal plasma concentration
- M/P: Maternal milk / plasma ratio AUC (i.e. Fraction of drug expressed in breast milk)
- V: Volume of milk ingested by infant
- Arbitrary cut-off ***10% as “safe” drug
Breastfeeding concerns:
1. Transfer of drugs is influenced by protein binding, ionisation, lipid solubility of drug (all lipid soluble drugs will inevitably appear in breastmilk)
2. Topical preparations, sprays, inhalational agents, regional analgesia expose less amount to infants than parenteral administration
3. Infants (esp. premature babies (∵ immature BBB, immature liver)) have lower drug clearance than adults
4. Feeding immediately ***prior to a dose may help to minimise infant exposure
Labour pain
1st stage:
- Visceral C + A-delta
- Lower segment + Cervix —> Paracervical ganglion —> Hypogastric nerve —> Lumbar sympathetic chain —> T10-L1 dorsal root ganglion + Contralateral —> Supraspinal
2nd stage:
- Larger A-delta fibres (need to give larger concentration of LA to block the nerves —> need to give non-epidural analgesia)
- Vagina, Perineum —> Pudendal nerve —> Somatic S2-4 dorsal root ganglion
Why pain relief is important?
- Extreme pain
—> May increase postnatal depression
—> Predict PTSD
—> Impair cognitive function
Labour pain relief methods
Non-pharmacological:
1. Moxibustion
2. Massage
3. Support
4. Birth ball / positions
5. Water bath
6. Intracutaneous water papules
Pharmacological:
1. Systemic medication
- Inhalational
—> Entonox
—> Volatile agents
- Parenteral opioids
—> IM injections
—> IV PCA Remifentanil
2. Neuraxial analgesia
Evidence-based:
What works:
1. Epidural
2. Combined spinal-epidural (CSE)
3. Inhaled anaesthesia
What doesn’t work:
1. Hypnosis
2. Biofeedback
3. Sterile water injection
4. Aromatherapy
5. **TENS (Transcutaneous electrical nerve stimulation)
6. **Parenteral pethidine
Entonox
- ***Nitrous oxide:O2 = 1:1
- ***Not interfere with uterine activity
- Require one way valve + scavenging system
- Breathe in at beginning of contraction and continue until end of contraction
- ***Environmental pollution (Greenhouse gas)
- Inhibition of Methionine synthase (enzyme involved in DNA synthesis) after prolonged administration —> unsure whether will cause harm to baby
- SE: ***Dizziness, stopped once analgesic is stopped (SpC PP)
Systemic opioids
- IM Pethidine
- **Poor efficacy
- Maternal **drowsiness + **nausea
- **Metabolite Norpethidine has prolonged t1/2 16-20 hours
—> ***Neonatal respiratory depression
—> Affect neurobehavioural scores of newborn - IV PCA Remifentanil infusion
- PCA equipment
- Need for equipment to treat **complications of systemic opioid
—> Supplementary O2
—> SpO2
—> **Capnography
—> ***Naloxone
- Training
Epidural analgesia
Unique advantage:
1. Efficacious pain relief in experienced hands (Gold standard for comparison)
2. Versatility
- Allow top up if operative delivery needed
- Can convert analgesia to anaesthesia for operative delivery
- Avoid GA + maternal airway manipulation
3. Superior maternal haemodynamics
- Pre-eclampsia
- Maternal heart diseases
Extra benefits:
1. ↓Maternal plasma Catecholamines + related rise in maternal O2 consumption
2. ↓Maternal hyperventilation / hypoventilation associated with painful uterine contractions —> Improve uterine blood flow with a stable plasma CO2 level —> avoid vasoconstriction of uteroplacental arterioles
Regional techniques for analgesia / anaesthesia
- Spinal (aka Intrathecal / Subarachnoid)
- **rapid onset
- sacral analgesia
- technically simple
- **low drug dose
- **limited duration
- **dural puncture —> **headache, **infection risk - Epidural (aka Extradural)
- drug act by **diffusion to nerve roots via dura / absorbed by epidural fat (act as depot —> slow release of drug) / absorbed into systemic circulation
- **no dural puncture needed
- **flexible in duration
- **continuous technique (catheter, allow longer period of administration, more adaptable if long operation)
- **less hypotension
- **less motor block
- **slow onset
- **large drug volume compared to SA —> **maternal toxicity, **fetal drug exposure
- timing with anticoagulation (∵ epidural cannot be removed immediately after anticoagulation) - Combined spinal-epidural (CSE)
SE:
Procedure-related:
1. **Maternal hypotension (∵ sympatholysis, treated by vasopressor)
2. **Motor block (if significantly high concentration of LA used)
3. **Dural puncture (1%) / related headache
4. High block (uncommon)
5. Failure
6. **Urine retention
7. Epidural haematoma (rare)
8. Epidural abscess (rare)
Drug-related:
1. LA toxicity (accidental intravascular injection (rare))
2. N+V (opioid-related)
3. Pruritus (opioid-related)
4. Sedation (opioid-related)
5. Anaphylaxis
CI to regional anaesthesia / analgesia:
Absolute:
1. **Coagulopathy
2. **Systemic septicaemia
3. **Hypovolaemia (∵ Maternal hypotension)
4. **Raised ICP
5. Patients’ refusal
Relative:
1. Difficult anatomy
2. Pre-existing neurological deficits
Common Epidural / Spinal regimen in QMH
Spinal for C/S:
- 0.5% Heavy Bupivacaine + Preservative free fentanyl + Preservative free morphine
Epidural for C/S:
- 0.5% Bupivacaine
or
- 1% Ropivacaine
or
- 2% Lignocaine with 1:200000 adrenaline
Epidural for labour pain relief:
- Initiation bolus: 0.3% Ropivacaine
- Maintenance: 0.15% Ropivacaine + Preservative free fentanyl in PCA mode
Common operations performed in Obstetrics
- C-section
- Manual removal of placenta
- Cerclage (reinforce cervical muscle to prevent preterm birth)
- Perineal tear repair
- Instrumental deliveries
- Non-obstetric operations (e.g. appendectomy, endoscopy)
Advantages of regional anaesthesia
General:
1. Flexible in operation duration
2. Flexible in significant blood loss
3. Easier for invasive monitoring
Regional:
1. Avoid airway manipulation
2. Avoid aspiration risks
3. Avoid surges in BP
4. Avoid interaction with Mg
5. Avoid uteroplacental blood flow drops with catecholamines
6. Avoid infant drug exposure —> Avoid neonatal respiratory depression, Avoid neurotoxicity to fetal brain
8. Better maternal post-op analgesia
9. Better maternal post-op neurological monitoring
10. Earlier maternal post-op mobilisation
11. Encourages breastfeeding
General anaesthesia
Indications:
1. **Crash delivery
2. **CI to regional techniques
3. ***Failed regional techniques
Preparation before intubation:
1. Supplemental 100% O2 with tight sealing mask
2. IV access
3. Standard monitoring (including for baby)
4. Patient positioning: Sniffing air position unless CI
- most widely used position for ET intubation
- flexion of neck (Cervical spine) + extension of head (Atlanto-occipital joint)
—> head on 7-9 cm pillow / firm surface
—> horizontal alignment of **external auditory meatus + **sternal notch
—> align axis of **line of sight with **laryngeal vestibule axis —> easier direct laryngoscopy
5. Left lateral tilt / Manual displacement of uterus to avoid aortocaval compression
6. Start CPR + resuscitation if arrested
Airway assessment:
1. Short thyromental distance
2. Receding chin
3. Short incisor gap
4. Poor dentition
5. Limited Head + Neck movement
6. Obesity
7. Congenital craniofacial abnormalities
8. Mallampati classification
Problems in labour:
1. Airway edema as labour progresses (∵ Fluid administration + Valsalva maneuver) —> more difficult intubation
